The invention relates to a magnetic-field-dependent inductive proximity switch having a coil arrangement with a ferromagnetic core which is saturation-sensitive with respect to the magnetic field to be picked up and having an electronic analyzing circuit for triggering a switching process at a certain core saturation.
A magnetic-field-dependent switch of this type is known on the basis of FR-OS No. 24 25 648. There the magnetic core is developed as a closed toroidal core, that is enclosed by a coil supplied with an alternating current of about 50 ma and a frequency of 10 kc/s. By means of an approaching permanent magnet, the toroidal core is driven into saturation and the inductivity is thus changed. The drop in voltage at the ends of the coil that is dependent on the inductivity is used as a signal to trigger a switching process. It is true that this switch can be triggered by a permanent magnet through a non-ferromagnetic wall, but the switch requires a high supply current. For that reason, it cannot, for example, be used in the case of complicated automatic manufacturing equipment having many hydraulic or pneumatic devices and often several hundred proximity measuring points. In addition, the construction of switches of that type is complicated.
For proximity measuring points, inductive proximity switches are known having a high-frequency oscillator, the high-frequency field of which forms a sensor field for an approaching metallic tripping device. Energy is removed from the high-frequency field by means of the entering tripping device, resulting in a damping of the oscillatory circuit and thus, in a change of the control voltage potential for the electronic switch. In the case of such proximity switches, known, for example, on the basis of DE-PS Nos. 19 24 279 and 28 27 951, however, tripping cannot take place through a metal wall, for example, through the wall of a V2A-steel (a stainless non-magnetic steel) container, because such a metal wall is no longer penetrated by the electromagnetic high-frequency field to an extent that, on the back of the metal wall, a damping or detuning of the oscillatory circuit of the proximity switch could be caused by means of a metallic lug or similar device.
On the basis of DE-PS No. 27 39 967, a proximity switch is known where the oscillatory circuit of a high-frequency oscillator is arranged in a housing in such a way that metals or other objects only cause very little damping or detuning that is not sufficient to trigger a switching process. The tripping there takes place by the proximity of a special oscillatory tripping circuit tuned to the high-frequency oscillator, whereby it is ensured, in that case, that no switching process can be triggered by arbitrary foreign objects. This known proximity switch, comprising two high-frequency oscillators, also cannot be used as a switch that operates through a metal wall since the high-frequency fields on both sides would be damped or cut off in an interfering manner by means of such a metal wall.
From DE-PS No. 28 29 880, an electronic non-contact safety switch having an oscillator that can be influenced from the outside is also known, where the tripping element is a dish-shaped ferrite core forming the counterpart to the ferrite core of the oscillator. In this case, the reaction impedance is adjusted so that the amplitude of the oscillation is very small. When a metal part enters into the high-frequency field, the oscillations break off in the conventional manner which, in this case, however, is not used to generate a signal. When, in this case, the tripping ferrite core enters the high-frequency field, the amplitude of the oscillation rises so much that an output relay responds. This safety switch also cannot be used in cases where switching operations are to be triggered through a metal wall.
The invention is based on the task of creating a magnetic-field-dependent switch with a low power requirement, for example, in the range of from 0.1 to 5.0 milliampere, by means of which a magnetic field is picked up through a non-ferromagnetic wall and can be utilized for triggering switching operations.
In order to solve this problem, it is provided, according to the invention, that the coil arrangement is the inductive element of a high-frequency oscillatory circuit, the high-frequency oscillatory field of which, in the direction of the side of the magnetic field to be picked up, is essentially enclosed within the switch, and the amplitude of oscillation of which, which depends on the degree of core saturation, is picked up by the analyzing circuit which triggers the switching process when an amplitude threshold is not reached.
The switch according to the invention has the advantage that it can be manufactured cost-effectively and requires a low supply current in the range of 0.1 to 5 milliampere. Since the oscillator vibrations suddenly break off at a certain core saturation, the switch can be constructed to have high response precision. Since the switch can be operated through non-ferromagnetic walls, it offers new possibilities for usage as a proximity switch.
In the case of the switch according to the invention, the oscillatory circuit preferably has a coil-closing shell-type core or ferrite pot having center pockets as they are known in the conventional inductive non-contact proximity switches. According to the invention, the enclosure of the high-frequency oscillatory field can be achieved in various ways. According to one embodiment, the ferrite pot, at its open side, may be closed by a diametral short-circuit yoke. Said short-circuit yoke may consist of an identical ferrite pot, with both ferrite pots, with their open sides being placed against one another. However, the short-circuit yoke may also be plate-shaped, project above the ferrite pot on both sides, and provided with widened ends for the intensified trapping of an external magnetic field, and in its central area, may have a narrowing point of the cross section. An especially saturation-sensitive point, therefore, exists at this narrowing point of the cross section in the closed magnetic circuit.
As an alternative, the enclosure of the high-frequency oscillatory field may also be achieved by the fact that the oscillatory field of the oscillatory circuit is protected toward the outside against the effect of metals by a large mechanical casing. In this case, the ferrite pot may, for example, with its open side, point in the direction of the inside of the housing, in which case, its bottom will then point to the side of the magnetic field to be picked up. According to the invention, it is also possible to dispose the magnetic core eccentrically in the mechanical casing so that, in one or several approach (proximity) directions, it can be driven into saturation by a trip magnet.
According to another characteristic of the invention, the magnetic core may also be provided with one or several constrictions for the local increase of the magnetic flux density. Advantageously, the magnetic core may be provided with said constriction at the transition from the central part to the bottom. However, it is sufficient if only a small partial area of the oscillatory magnetic field circuit is brought into saturation.
In a further development of the invention, it may be provided that an external electromagnet or permanent magnet is disposed at a fixed distance is assigned to the switch as a trip magnet, and that a movable ferromagnetic short-circuiting bridge for the shielding or release of the magnetic field of the trip magnet is provided between the switch and the trip magnet as the actual switching trigger.
According to the invention, the magnetic-field switch may, at the same time, be developed as an inductive proximity switch that responds to the approach of a metal lug at its active surface, with said proximity switch, with another surface than the active surface, being able to be mounted on a non-ferromagnetic metal wall without affecting the oscillatory circuit, in which case, a tripping device for the approach to that other surface in the form of an electomagnet or a permanent magnet is then assigned to the proximity switch, with the electromagnet or permanent magnet being of such intensity that, at a certain proximity, it drives the magnetic core into saturation. In this case, the switch may be designed for an OR-operation or for an AND-operation. In the latter case, the switch will only trigger a switching process if an approach of metal takes place on the one side, and, at the same time, the approach of a sufficient magnetic field takes place on the other side. An AND-operation is advantageous in the case of safety circuits, for example, in the case of presses or elevators.
By means of the invention, the effect of external magnetic fields which, until now, in the case of inductive proximity switches, has only been known as a dangerous interfering influence, is being utilized in order to be able to dampen the oscillator circuit through a separating metal wall for the purpose of triggering directed switching processes. In regard to its operational possibility, the switch according to the invention can be compared with a Reed contact switch, in which case, the disadvantages of the Reed contact switch, namely, a possible faulty operation in the case of mechanical vibrations or a possible sticking of the contacts, however, are avoided. When using a permanent magnet, it is advantageous that it is a passive part, in this case, that does not require its own power supply. Another significant advantage of the magnetic-field switch according to the invention is the fact that it, if it is only to be operable by a trip magnet or an external magnetic field, can now be completely enclosed in a metal housing, for example, consisting of corrosion-resisting V2A-steel, so that the switch can be made completely insensitive with respect to attacks by corrosion, flying sparks, high hydraulic pressures, etc. The switch is also insensitive to magnetic overload. For the construction of the magnetic switch according to the invention, practically any known type of inductive proximity switch can be used as a basis, for example, direct-current (DC), alternating-current (AC), alternating-current/direct-current or NAMUR-types.
Finally, the switch according to the invention may also be used as excess current delay (overload relay) in the case of high-voltage lines.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.